Field Effect Transistors (FETs) have traditionally been built with a gate node, a source node and a drain node. Metal Oxide Semiconductor FETs (MOSFETs) are presently the most commonly manufactured type of transistor. FIG. 1 shows a gate node 101 for a MOSFET. The gate node 101 is comprised of a metal and/or (more commonly) a heavily doped polycrystalline silicon layer which behaves like a metal. The gate node 101 is separated from an underlying conductive semiconductor region 103 by an oxide layer 102. The gate node 101, oxide 102 and conductive semiconductor region 103 essentially form a capacitor structure.
The electric field strength within the oxide layer 102 is proportional to the voltage between the gate node 101 and the underlying semiconductor wafer; and, the electronic field strength within the gate oxide layer is inversely proportional to the thickness of the oxide 102. Thus, the higher the gate node 101 voltage and the thinner the oxide layer 102, the greater the electric field strength. If “too strong” an electric field is established within the oxide layer 102, the oxide layer 102 will suffer “dielectric breakdown”.
Dielectric breakdown is a form of oxide layer 102 damage. An oxide layer 102, being a dielectric layer 102, is an electrical insulator rather than an electrical conductor. As such, only an infinitesimal DC current IOX (e.g., a few nanoamps (nA) or picoamps (pA)) will flow through oxide layer 102 if a voltage below a critical voltage at which dielectric breakdown occurs is applied to the gate node 101 and the oxide layer 102 has not already suffered dielectric breakdown. Because of the infinitesimal current, the DC resistance ROX of the oxide layer 102 is said to be “near-infinite” (e.g., tens or hundreds of Megohms (MΩ)).
If the oxide layer 102 experiences dielectric breakdown, however, the behavior of the oxide layer 102 thereafter changes from that of an insulator to that of a semiconductor. Essentially, the DC resistance ROX of the oxide layer 102 drops from its pre-breakdown value to a smaller value so as to allow a more substantial current such as tenths of microamps (μA) or higher.
Traditionally, the largest voltage that could reasonably be applied to a semiconductor chip's transistors has been well beneath the critical voltage at or above which dielectric breakdown could occur. With the continued miniaturization of transistor sizes and corresponding reduction in oxide thickness, however, it is presently more feasible to apply a gate voltage above a critical threshold value at or above which dielectric breakdown will occur.